The practice of cementing wells to provide isolation between exposed formations along a wellbore has been used in the drilling industry for a century. It is estimated over a billion sacks of cement have been used for this purpose. The process of primary and remedial cementing has been extensively studied over the past six decades and many improvements have been made which increase the effectiveness of the process in order to achieve zonal isolation. However, there are several fundamental limitations in the current state of the art of cementing wells that limit success in providing the most effective zonal isolation.
One limitation is that it is difficult to completely remove or displace all the drilling fluid (or other fluid) in the wellbore with cement slurry when placing the cement during the primary cementing process. Another limitation is that the cement does not bond (or adhere) satisfactorily to casing surfaces and drilling fluid filter cake or residue. Furthermore, changes in the stress state of the wellbore during completion of drilling and throughout the productive life of the well can damage the cement bond/seal to cement and formation. These changes in the stress state of the wellbore may be the result of thermal changes (temperature) or pressure changes (for example, displacing a heavy fluid in the casing with a lighter fluid during completion of the well or additional deepening of the well, or changes in formation pressures or changes in annular pressures).
It is known in the art to include additives to prevent fluid loss in cement compositions. U.S. Pat. No. 4,716,965 discloses an improvement in a cementing process for preventing fluid migration between the casing and cement in a situation where a casing is suspended within a well and a slurry of cement is flowed into the space between the casing and the borehole wall and allowed to harden, comprising: surrounding at least one portion of the outer surface of the casing with a self-supporting sheath of an elastomeric foam comprising alternately arranged layers of a closed cell polyurethane foam and a closed cell polyethylene foam which, together, are capable of remaining resilient and retaining the structural integrity of the sheath after compression by the hydrostatic pressure of a slurry of cement; inflowing the cement slurry into the borehole around the casing and sheath; and allowing the cement to harden with the resilient tendency toward expansion of the elastomeric foam ensuring good adhesion of the sheath to both the casing and cement.
U.S. Pat. No. 5,151,131 discloses a liquid fluid loss control additive for an aqueous well cement composition, said additive including an organophilic clay suspending agent present in an amount in the range of from 0.5 to about 8 percent by weight of said liquid hydrocarbon, a surfactant present in said additive in an amount in the range of from about 0.5 to about 8 percent by weight, and at least one hydrophilic polymer present in an amount in the range of from about 40 to about 150 percent by weight of said liquid hydrocarbon.
U.S. Pat. No. 5,458,195 discloses an improved cementious composition which can include drilling fluid as a component and methods of cementing wells utilizing such compositions, said cementious composition including a drilling fluid present in an amount up to about 70% by volume, and a hardenable resinous material selected from the group consisting of vernonia oil, epoxidized linseed oil or soy oil, an acrylic resinous material, an epoxy resinous material, a phenolic resinous material and mixtures of said resinous materials present in said composition in an amount in the range of from about 1% to about 50% by weight of said cementious material or materials; and water present in said composition in an amount in the range of from about 20% to about 175% by weight of said cementious material.
In the current state of the art there are limitations on the effectiveness of cement slurries and drilling fluids that result in less than desirable adherence of cement to casing surfaces and drilling fluid cake or residue. Many of these problems could be effectively addressed by cementious compositions characterized by improved ductility and by cement slurries and drilling fluid formulations that afford better bonding, sealing and adhesion.